Open bottle half-full of rose champagne in my fridge sealed with stopper, for a week...can I use for poaching fruit? How long before tossing?

Add to your preserves if you are making jams or conserves, or chutneys.

A rosé-raspberry granita has a nice ring to it....

sounds like a fabulous choice for poaching fruit. I'd also probably reduce the poaching liquid to make a memorable and unique sauce for the results. Pears? [You're talking to the "Champagne Lassie" here... :D

Once it's been opened and left for more than a number of hours (unless closed under a truly superior closure/protection system) it's finished as far as its original intent. At this point it just doesn't matter; you could keep it for weeks before using as a poaching liquid. (I'm finishing WSET Diploma and work in the drinks industry.)

Toss it?? No! I keep containers of leftover wine in my fridge to be used for cooking purposes indefinitely. If I had room in my freezer, I would freeze them. But after only a week? Poach away! Sounds delicious!